In Orthogonal Frequency Division Multiple Access (OFDMA) systems based on a multi-carrier scheme, resources are allocated in a unit of subchannels consisting of subcarriers. That is, a whole subcarrier is divided to be shared by a plurality of users, and thus a multi-user diversity gain can be ensured in a frequency domain. In a broadband wireless communication system, the same frequency is reused by cells. According to a received signal strength indication (RSSI) and interference between neighboring cells in case of reusing the same frequency, an adaptive modulation and coding (AMC) scheme is applied to maximize a throughput.
FIGS. 1A and 1B illustrate an example of cell frequency arrangement and frequency reuse. The same frequency is repeated and reused with a specific distance, and this is called “frequency reuse”. A frequency reuse rate K is defined as a value indicating the number of cells using the same frequency, and is an important concept in a mobile communication system. The greater the frequency reuse rate, the farther the distance between cells using the same frequency, and thus the less the interference caused by the use of the same frequency.
FIG. 1A illustrates a case where the frequency reuse rate is three (3), wherein three frequencies are reused with a specific distance. FIG. 1B illustrates a case where the frequency reuse rate is seven (7), wherein seven frequencies are reused with a specific distance.
If the frequency reuse rate is one (1) in a system, interference between neighboring cells is significant in a boundary of a cell (or a sector). Thus, throughput deterioration is inevitable. As a method for improving performance in the cell boundary when using the frequency reuse rate of ‘1’, a fractional frequency reuse (FFR) scheme can be considered in which a whole subcarrier is orthogonally divided into a plurality of subbands, and these subbands are properly arranged so that some of the subbands are unused by each cell, thereby reducing the same channel interference between the neighboring cells.
Mobile stations (MSs) are generally located in a center area and a boundary area of a cell, and are differently affected by interference caused by neighboring cells. This feature is used in the FFR scheme. That is, an MS located in the center area of the cell is close in distance to a base station (BS), and thus a path loss does not have a great effect on decrease in an RSSI of a signal component. However, since the MS is relatively far from a neighboring interfering BS, the MS is highly affected by the path loss and thus influence of a co-channel interference (CCI) is eventually decreased. On the other hand, an MS located in the boundary area of the cell is separated by a similar distance from both a serving BS and the interfering BS, and thus a signal component and an interference component are received in the same RSSI. Therefore, influence of the CCI is increased. Accordingly, in the FFR scheme, the MS located in the center area of the cell uses a resource having a frequency reuse rate of ‘1’, and the MS located in the boundary area of the cell uses a resource having a frequency reuse rate greater than one (1). As a result, reception throughput of the MS located in the boundary area is ensured to some extent.
As described above, in the FFR scheme, in order to ensure reception performance in the boundary area of the cell, a resource having an increased frequency reuse distance is allocated. However, the increase in the frequency reuse distance results in decrease in an amount of radio resources available in each cell. As a result, capacity is reduced in comparison with a system having a frequency reuse rate of ‘1’. For example, if a resource having a frequency reuse rate of ‘1’ and a resource having a frequency reuse rate of ‘3’ are equally divided in 1:1, and in this state, if a resource having a frequency reuse rate of ‘3’ is equally divided into three (3) parts, then an amount of frequency resources available in each cell is reduced to two thirds (⅔) of the case of using a frequency reuse rate of ‘1’. As such, the use of FFR scheme ensures a throughput of the MS located in the boundary area of the cell but disadvantageously decreases an amount of frequency resources that can be used in a BS. Therefore, there is a problem in that an overall cell capacity is reduced.
In order to avoid the reduction of cell capacity, a frequency reuse distance needs to be ensured. For this, a restricted band which is unused in each cell can be assigned to the MS located in the center area of the cell with a low transmission (Tx) power. That is, when the Tx power of the restricted band is maintained to be sufficiently low, the Tx power can be allocated to an MS located close to a BS in a condition that a CCI is not significantly generated. Therefore, resources can be further effectively used. Accordingly, there is a need for an apparatus and method for maintaining a CCI to a specific level in a restricted band.